Packaged magnesium anode with cemented backfill



Patented Jan. 24, 1950 PACKAGED MAGNESIUM ANODE WITH CEMENTED BACKFILLHerman H. Miller, Midland, Mich., assignor to The Dow Chemical Company,Midland, Mich., a corporation of Delaware Application July 31, 1947,Serial No. 765,048

I This invention relates to improved packaged anodes forthe. galvanicprotection. of underground metals. 'It also concerns protection systemsusing such-anodes.

. ,In galvanic systems for thecathodic protection of pipelines and.other underground structures, sacrificial electrodes of a metal anodicto the structurasucli as magnesium, are buried in the earth nearthestructureand are connected to itfby electrical conductors. Theresulting flow of current maintains the structure cathodic with respectto thesoil and greatly minimizes its corrosion. Since magnesium tends tobe. consumed uselessly when indirect'contact'with some types of soils,it is customary, in using this Illetalas a sacrificial anode, to 'buryit in -a prepared bed or backfill. designed to control the chemicalnature of the anode environment.

In prior. practice, such backfills have consisted of powder mixtures, e.g. of gypsum and bentonite, which have been tamped in place around the.anode when it is being buried. In general, these backfill is associatedwith a magnesium anode in a unit package which can be installed simplyby burying it. Another object is to provide a packa edanode in whichloose powder is not present and a special container is not required, thebackflll itself having a fixed'formand being of sufficient mechanicalstrength to withstand'stresse incident to shipment and installation. Theinvention will be explained with reference to the accompanying drawing,.inwhich Fig. 1 is a schematic vertical section showing the manner "(ifusing one form of the new packaged anode in the galvanic protectionof aburied pipeline;

.Fig. 2 is a vertical section through another form of packaged anodealong the line. 2-2 in Fig. 3; and

Fig. 3 is 'a horizontal section through'the article of Fig, 2 along theline 33 in Fig. 2.

In the packaged anodes of theinvention, the .magnesium metal anodei'simbedded in a unitary self-sustaining backfill' consisting 'of acemented mass of a finely-divided or granular non-acidic solidelectrolyte. In efiect, then, the anode metal encased in a body ofporous stone-like material which is of such composition and size as to 6Claims. (01. 204-197) 2 provide a suitable electrolytic environment forthe anode.

In so far as known, any granular or powdered non-acidic solidelectrolyte may be used in making the new packaged anodes, since themajor func tion of this material is simply to render the backfillelectrolytically conductive. In general, alkaline earth metal hydroxidesand alkalior alkaline-earth metal salts of acids which form watersolublemagnesium salts, or mixtures of these substances, are preferred. It isparticularly advantageous to select an electrolyte which is onlysparingly soluble'in water, i. e. one having a solubility from about 0.1to about 2 percent by weight at atmospheric temperatures. Calciumsulfate, usually in the form of gypsum (CaSOaZI-IZO) or calcined gypsum,is perhaps the best choice because of its low cost and excellentperformance, although magnesium sulfite is also very satisfactory.

The material'used to bind together the particles of finely-dividedelectrolyte in making the new packaged anode may be any non-acidicinorganic cement capable of setting in the presence of water. Plaster ofParis (calcined gypsum), hydraulic cements; such as Portland cement, andmagnesia cements, especially magnesium oxysulfate and oxychloride, arepreferred. The proportion of cement used is not critical, but should besufficient to insure that the electrolyte is cemented together to aunitary mass of good strength. In general, the cement should constituteat least 10 percent by weight, and better 20 to 30 percent, of the totaldry constituents of the backfill.

In addition to electrolyte and cement, the backfill composition for thenew packaged anodes may also contain minor proportions ofother nonacidicmaterialaefg. sand, gravel, crushed washed coke; and otherelectrolytically inert fillers. Granular magnesium hydroxide or oxidemay be added, each having the desirable effect or assisting inmaintaining alkalinity. Powdered or granularvbentonite-,(montmorillonite, a sodium-type base-exchange. volcanicclay) is also advantageous in that it tends to absorb and retain waterand to retard leaching of the electrolyte.

In a preferred method of making packaged anodes according to theinvention, the electrolyte, cemenuand filler, if any, are separatelyground to coarse powder form and are then mixed. The mixture is thenstirred into water to form a thick slurry, which .is poured around themagnesium anode while the latter is centered in a .mold. After theslurry has set, the mold may be taken off, leaving the packaged anodeready for use. It is desirable, to secure a cemented backfill of goodporosity,-to use rather coarsely ground materials.

While, in the foregoing, the electrolyte and the cement have beendiscussed separately, it is entirely possible for a. single substance tofunction as both. For example, it is regarded as within the invention toimbed a magnesium anode in a set mass of plaster of Paris or ofmagnesium oxysulfate, in which case no other electrolyte or filler needbe added.

A typical installation for cathodic protection, using a packaged anodeaccording to the invention, is shown in Fig. 1, in which a steelpipeline 4 buried in the earth is being protected. The packaged anode 5is made of an elongated cylindrical body of magnesium 6 cast around acore formed by part of a steel cable I, the other portion of whichextends beyond the anode to con-1 stitute an electrical lead-wire. Themagnesium -& is imbedded in a backfill consisting of a cemented block 8formed by ground gypsum bound together by magnesium oxysulfate. Asshown, the block 8 and its contained anode 6 are buried in the earthnear the pipeline 4, with the cable 1 being connected electrically tothe pipe by a conductor 9.

In making the installation, a suitable hole is dug, the packaged anode 5is lowered in place, and earth is tamped around it. The electricalconductor to the pipeline is then installed and buried. In dry soils,the start of electrolytic action may be hastened by pouring water aroundthe spot where the anode is buried.

In an alternative way of making the new packaged anodes, the backfillmay contain reinforcing to strengthen the cemented mass. Preferredreinforcing materials are flexible non-conductors, most suitably porousfabrics such as burlap or canvas. A typical construction of this sort isshown in Figs. 2 and 3 in which the magnesium anode E is imbedded in amass In of ground gypsum cemented together by Portland cement which isreinforced by burlap ll wrapped spirally around the anode.

In making the packaged anode of Figs. 2 and 3, a suitable length ofburlap is laid on a flat surface and a settable mixture of gypsum andPortland cement is spread over it as a thick layer. The anode is thenlaid on top at one end and is rolled along the surface to wrap theburlap and cement mixture around the anode. After the cement has set,the article is ready for use.

In field use of the invention, the number and size of anodes and thequantity of backfill required to secure effective cathodic protection ofa given pipeline or other structure are determined by well-knownengineering principles.

The following examples will further illustrate the invention.

Ewample 1 Packaged anodes were prepared in the manner illustrated inFigs. 2 and 3, using magnesium rods as anodes and canvas as reinforcing.The

backfill consisted of a mixture of: ground gyp- 7 Packaged anodes weremade up as in Example 1, except that the cementable backfill mixturewas: ground gypsum, '70 parts, and magnesium sulfite 5 parts aselectrolytes; ground bentonite, 5 parts, as filler; and magnesium oxide,10 parts,

and magnesium sulfate, 5 parts, as cement.

I Example 3 Packaged anodes were made up as in Example 1, except thatthe backfill mixture was: ground gypsum, 20 parts, and magnesiumsulfite, 2 parts as electrolytes; silica sand, 58 parts, as filler; andmagnesium oxide, 20 parts, and magnesium sulfate, 8 parts, as cement.

Example 4 Packaged anodes were made up as in Example 1, except that thebackfill was: ground gypsum, 40 parts, and magnesium sulfite, 2 parts,as electrolytes; silica sand, 36 parts, and bentonite, 2 parts, asfillers; and Portland cement, 20 parts, as cement. This mixture waswetted with a 12 B. magnesium sulfate aqueous. solution instead ,of withwater when it was being applied to the anodes.

All of these anodes were tested in the cathodic protection of iron innatural water and developed adequate currents at good electrochemicalefiiciencies.

While the invention has been described as useful in the cathodicprotection of underground ferrous metal structures, it is applicable inprotecting structures of any corrodible metal cathodic to magnesium. Thesacrificial anodes may be either of magnesium or of a magnesium-basealloy, both being comprehended by the term magnesium meta as used in theclaims.

What is claimed is:

1. A packaged anode for use in cathodic protection systems comprising amagnesium metal anode imbedded in a self-sustaining porous mass of agranular electrolyte selected fromthe class consisting of alkaline earthmetal hydroxides and alkaliand alkaline-earth metal salts of acids whichform water-soluble magnesium salts cemented together by magnesiumoxysulfate.

2. A packaged anode according to claim 1 wherein the electrolyte has asolubility in Water of from 0.1 to 2 percent by weight at atmospherictemperatures.

3. A packaged anode according to claim 1 wherein the electrolyte is'calcium sulfate.

4. A packaged anode for use in cathodic protection systems comprising amagnesium metal anode imbedded in a set mass consisting essentially ofgranular ypsum cemented together by magnesium oxysulfate.

5; A packaged anode according to claim 4 wherein the set mass alsocontains a small proportion of bentonite.

6. In combination with an underground structure of a corrodible metalcathodic'to magnesium, a cathodic protection system comprising apackaged anode as defined in claim 1 buried in the earth near thestructure and electrically connected thereto.

HERMAN MILLER.

REFERENCES CITED The following references are of record the file of thispatent:

UNITED STATES PATENTS Y Date Number Name 489,668 Bryan Jan. 10, 18932,088,307 Ruhofi et al July 27, 1937 OTHER REFERENCES quist, April 1944,page 83.

1. A PACKAGED ANODE FOR USE IN CATHODIC PROTECTION SYSTEMS COMPRISING AMAGNESIUM METAL ANODE IMBEDDED IN A SELF-SUSTAINING POROUS MASS OF AGRANULAR ELECTROLYTE SELECTED FROM THE CLASS CONSISTING OF ALKALINEEARTH METAL HYDROXIDES AND ALKALI- AND ALKALINE-EARTH METAL SALTS OFACIDS WHICH FORM WATER-SOLUBLE MAGNESIUM SALTS CEMENTED TOGETHER BYMAGNESIUM OXYSULFATE.